It is well known that conventional SiO.sub.2 -based single mode optical fiber generally has minimum loss at about 1.55 .mu.m and zero chromatic dispersion at about 1.2 .mu.m. Although it is possible to design optical fiber to have both minimum loss and zero chromatic dispersion at about 155 .mu.m (such fiber is generally referred to as "dispersion-shifted" fiber), most presently installed single mode fiber is of the conventional type and is operated at a signal wavelength of about 1.3 .mu.m, at or close to the zero chromatic dispersion wavelength of the fiber.
Considerable advantages could be realized if already installed optical fiber systems could be operated at 1.55 .mu.m. These advantages include the possibility of increased distance between repeaters (due to the lower fiber loss), and the possibility of using Er-doped fiber amplifiers (EDFAs) instead of repeaters. However, straight-forward upgrade by changing operating wavelength is not practical, due to the significant amount (exemplarily about 17 ps/nm.multidot.km) of chromatic dispersion of conventional SiO.sub.2 -based single mode fiber at 1.55 .mu.m The dispersion typically would limit such a system to a bit rate of at most about 2 Gbit/sec for distances of order 500 miles.
Several possible ways of dealing with the problem have recently been proposed. These can be divided into "active" and "passive" techniques. Herein we are only concerned with passive techniques, in particular, with techniques that involve use of dispersion-compensating (DC) fiber. U.S. Pat. No. 4,261,639 discloses dispersion compensation by catenation of single mode fibers of respectively positive and negative dispersion at the operating wavelength.
DC fiber not only can advantageously be used to upgrade an existing fiber optic communication system but could also be used in new systems (e.g., transoceanic submarine systems that use dispersion-shifted fiber) that are designed to operate at 1.55 .mu.m, e.g., to ease manufacturing specifications on the dispersion-shifted fiber.
C. D. Poole et al., (Optics Letters, Vol. 17(14), p. 985; see also co-assigned U.S. patent application Ser. No. 766,600, filed Sep. 26, 1991 now U.S. Pat. No. 5,185,827) disclose a DC fiber designed to support, in addition to the lowest order mode (LP.sub.01), also the lowest higher order mode (LP.sub.11), and also designed to have a cut-off wavelength for the LP.sub.11 (.lambda..sub.c11) mode just slightly above the operating wavelength (.lambda..sub.op). Typically, at the upstream end of the DC fiber a mode converter is provided to convert the LP.sub.01 mode into the LP.sub.11 mode, and at the downstream end of the DC fiber another mode converter is provided to convert the LP.sub.11 mode back into the LP.sub.01 mode. The technique of Poole et al. is able to compensate both dispersion and dispersion slope and can yield desirably large values of negative dispersion (absolute value&gt;100 or 150 ps/nm.multidot.km; exemplarily -228 ps/nm.multidot.m). However, the Poole et al. technique undesirably requires use of special components (mode converters and polarization rotators) which not only add cost but also typically introduce additional loss. Furthermore, typically only about 15-20% of the total power propagates within the core, owing to the proximity of .lambda..sub.op to .lambda..sub.c11, typically resulting in relatively high loss. Still furthermore, the prior art technique typically requires slightly elliptical DC fiber to remove the four-fold degeneracy of the LP.sub.11 mode.
A. J. Antos, (Proceedings, Conference on Optical Fiber Measurements, National Institute of Science and Technology, Colo., September 1992, p 89) discloses DC fibers that are designed to support only the LP.sub.01 mode and to have negative chromatic dispersion at 1.55 .mu.m. The disclosed DC fibers have relatively small negative chromatic dispersion (absolute value .ltoreq.100 ps/nm.multidot.km; exemplarily -65 ps/nm.multidot.km), necessitating the use of long lengths (e.g., 39km of DC fiber to compensate the dispersion of 150km of conventional fiber) of DC fiber. Furthermore, the Antos technique apparently is practical only for dispersion compensation, with dispersion slope compensation being considered ". . .not easily achieved in practice . . ." by the author.
In view of the considerable commercial significance of DC, a technique that avoids or at least mitigates the shortcomings of prior art DC techniques would be highly desirable. This application discloses such a technique, and articles that embody the inventive technique.